Varactor diode tuners have contributed to the simplification of tuning systems in general, and television receiver tuning systems in particular. In such tuners, which are often referred to as electronic tuners, the varactor diodes exhibit capacitance variations with changes in bias voltage and serve as the variable reactances in-otherwise-conventional tuned circuits. Such tuning systems are easy to tune, free from RF signal carrying contacts and afford the designer great versatility in receiver styling. As pointed out in the related applications, their most serious drawbacks are the limited range of diode capacitance change and the nonlinear relationship between frequency and bias voltage.
The invention in the first-mentioned related application (Ser. No. 791,897)--now U.S. Pat. No. 4,142,157--provides an attractive solution to these problems and the problems associated with implementation of the Federal Communications Commission so-called "equal tuning" rule for VHF and UHF television channels. In brief, that system produces a separate "slope factor" which is related to the slope of the tuning voltage-versus-frequency characteristic for proportioning the "fine" tuning voltage such that equal frequency excursions are experienced for equal tuning information increments. The result is a truly "equalized" tuning system. The slope factors are stored in appropriate channel-number-addressable memories as are the nominal (coarse) tuning informations and fine tuning informations. For each channel selection a nominal tuning voltage information, a fine tuning voltage information and a slope factor are produced. The fine tuning information is multiplied by the slope factor and combined with the nominal tuning information for conversion to the final tuning voltage.
The invention in the second-mentioned copending application Ser. No. 807,627 is concerned with memory utilization in digital tuning systems and the savings in memory which may be achieved by proper utilization of the slope factor. The structure of that invention accomplishes significant memory reduction by storing initial value tuning information for a pseudo channel in each frequency band and separate tuning increment information, representing the tuning voltage changes required to successively tune from one channel to the next, beginning with the pseudo channel. (These increments are the difference equation analog of the slope factors defined in the application Ser. No. 791,897--now U.S. Pat. No. 4,142,157--). Upon occurrence of a channel change, an arithmetic computation is performed in which the initial value information and successive increment informations are added. The initial value tuning information is selected at a point 6 MHz below the lowest numbered channel in the band which point is then referred to as the pseudo channel number. Thus, in the low VHF band, for instance, rather than storing complete information words corresponding to the nominal tuning information for channels 2-4, the nominal tuning information for pseudo channel 1 is stored along with the slope factors or increments required to go from pseudo channel 1 to real channel 2, from channel 2 to channel 3, and from channel 3 to channel 4. Suitable logic and apparatus are provided for summing the pseudo channel information and successive increments for obtaining the nominal tuning information corresponding to the selected channel number.
Since the last increment represents the slope factor of the tuning curve at the selected channel, and since this slope information is separately available, it is readily usable for equalization of any auxiliary tuning voltage source to provide true equalized tuning for the system. In the offset fine tuning system disclosed, one-half of the fine tune information, after equalization, is added to the derived nominal tuning information to produce the final tuning information for the selected channel.
There is no art known to the inventor which is relevant to the invention described and claimed; that is a system which "computes" a tuning voltage by algebraic summation of nominal tuning information for a reference channel and increment tuning information representative of tuning differences between channels.
The present invention represents a further improvement in memory utilization over that obtainable in the system of the Ser. No. 807,627 application. In essence, only the differences in increment tuning information from channel to channel are stored in the memory. Effectively these difference increments result from taking the "slope of the slope" of the tuning characteristic at each channel tuning position, and may be conveniently referred to as a second derivative system. Thus rather than storing a tuning information increment equivalent to a one volt tuning change for example, only the tuning increment change for that channel, which may amount to only a tenth of a volt, is stored and thus a substantial further savings in memory is obtainable. The nominal tuning information for the selected channel is derived by summing the nominal tuning information for the pseudo channel and first increment tuning information in that band with the appropriate number of successive second increment tuning informations. The first and second increment tuning informations are added to produce a last tuning increment for the selected channel, which is the slope factor, and which may be conveniently used to produce equalized tuning as disclosed in the Ser. No. 791,897 application.